The present invention relates to a tray for carrying a magnetoresistive head (hereinafter often referred to as xe2x80x9cMR headxe2x80x9d) of magnetic disks.
Trays used for carrying wafers, IC chips, and other electronic parts are required to have antistatic properties. Because of this, antistatic properties have conventionally been imparted to such trays by using as a molding material, a resin composition comprising a resin such as an ABS resin, and an antistatic agent or a conductivity-imparting ingredient (e.g., carbon black) dispersed therein.
However, the trays obtained by molding the resin composition containing an antistatic agent or a conductivity-imparting ingredient (e.g., carbon black) have had the following problems. Namely, the trays containing an antistatic agent have problems, for example, that the electrical conduction therein is apt to be influenced by the ambient humidity because the mechanism of electrical conduction is attributable to ionic conduction, that the antistatic agent flows out upon cleaning and during long-term use, resulting in reduced antistatic properties, and that incorporation of a large amount of an antistatic agent impairs heat resistance. The trays containing carbon black have the following problem. Although carbon black is uninfluenced by humidity, cleaning, etc., it should be added in a large amount for imparting conductivity. As a result, the surfaces of the moldings obtained have poor resistance to scratching and wearing and are apt to release wearing dust and carbon particles.
For solving these problems, a material obtained by adding carbon fibers to, for example, a polycarbonate is used as a material for trays for carrying magnetic heads of hard disks.
Incidentally, with the recent increase in the density of heads, MR heads are coming to be mainly used in place of conventional thin-film heads. An MR head comprises an part, an MR element attached to the tip of the arm part, and a lead wire connected to the MR element.
In contrast to the conventional thin-film element in which signals are detected by detecting the current generating when the coil approaches a signal magnetic field the MR element is a device in which a slight sensing current is caused to flow therethrough and a signal magnetic field is detected based on current resistance. Due to this mechanism, MR heads have exceedingly improved detection sensitivity and this enables media to have a reduced track pitch, i.e., an increased capacity. Recently, a GMR head has been developed which is intended to attain a further increase in capacity.
Since an MR head detects signals by the mechanism in which magnetism is sensed based on a resistance change of the slight current (sensing current) flowing through the MR element as described above, there is a high possibility that even a slight noise current might damage the MR element. Consequently, as compared with conventional integrated magnetic heads and ICs, MR heads are far more sensitive to an electrostatic discharge attributable to a tray/head potential difference and to a contact current generating upon head/tray contacting.
In a process for assembling an MR head, a lead wire is connected to an MR element and thin MR element having the lead wire is attached to the tip of an arm part. In this lead wirer which is a metal wire coated with a polyimide, the polyimide/metal wire contact area is always in a charge separation state due to the contact potential difference between the polyintide and the metal wire and is hence in an electrically unstable state. As a result, the contacting of the lead wire tip to a tray is apt to result in charge transfer in the contact area and this increases the possibility of damage.
For the reasons described above, trays for carrying MR heads have a serious problem that the MR element is damaged by the electrostatic discharge or excessive contact current which occurs or flows between the device and the tray or between a peripheral part and the tray due to the too low surface resistance of the tray.
Furthermore, in MR head assembly processes, MR heads in many cases are subjected, together with the tray, to cleaning and heating/drying. The tray is hence required not to foul or damage the heads during the cleaning and heating/drying. In particular, since the tray is exposed to a drying temperature exceeding 120xc2x0 C. in this drying, it is required to have such a degree of heat resistance that it can sufficiently withstand that drying temperature.
Incidentally, the performance primarily required of conventional antistatic or static-dissipating materials has been the ability to quickly eliminate static electricity generated by friction or contacting. Consequently, most of the related documents do not refer to a lower limit of resistivity (e.g., JPA-8-288266 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-W-8-508534 (the term xe2x80x9cJP-Wxe2x80x9d as used herein means an xe2x80x9cJapanese publication of a PCT applicationxe2x80x9d)) . With respect to cases where a high degree of static-dissipating properties are required as in IC trays, there is a description to the effect that a surface resistivity of 103 xcexa9/xe2x96xa1 or higher is desirable (e.g., JP-A-8-283584).
As described above, a polycarbonate/carbon fiber material has hitherto been used as the material of trays for carrying MR heads. However, this material has the following drawbacks and it has been difficult to use it as trays for carrying MR heads, which are especially sensitive to static electricity.
(1) Although carbon fibers can impart excellent conductivity when added in a smaller amount than carbon black, the composition tends to give moldings having a low surface resistivity. Because of this, a high surface resistivity cannot be realized which is required of MR head transfer trays. When the resistivity in increased by reducing the addition amount, it is difficult to obtain a molding having an even resistivity throughout because the carbon fibers come into an unstable state with respect to contacting with one another within the molding.
(2) The carbon fibers dispersed in a resin generally have a relatively large size with a fiber diameter of about froze 7 to 12 xcexcm and a fiber length of about from 50 to 300 xcexcm. Because of this, the composition gives a molding in which carbon fibers are exposed on the surface thereof. As a result, the surface of the molding has areas having an exceedingly low resistivity due to the exposed carbon fibers and electrically insulating areas consisting of the resin; these two kinds of areas each has a size of about from 10 xcexcm to 1 mm and are present in dispersed fashion. There is hence a high possibility that the sharp tip of the lead wire connected to an MR head might come into direct contact with an exposed part of the surface carbon fibers to cause damage due to an overcurrent. On the other hand, since the charges generated in the resin areas are less apt to be released, electrification occurs microscopically.
(3) In the step of subjecting MR heads as devices to ultrasonic cleaning with pure water or in other steps, carbon fibers themselves fall off the tray surface and resinous ingredients present among carbon fibers peel off. Such particles released not only cause head fouling or damage but also may come as foreign particles into the space between the head and a hard disk during the use of a hard disk drive and cause head crushing.
(4) In the case where carbon fibers are dispersedly incorporated into a resin, use is generally made of a binder for binding carbon fibers or a surface-treating agent for improving the dispersibility of the carbon fibers in the resin and improving strength at the interface therebetween. Use of these treating agents may pose problems that during cleaning with pure water, icons dissolve in the cleaning fluid (ion contamination), and that an organic compound having a relatively low molecular weight deposits on the device during heating (contamination with nonvolatile organics).
An object of the invention is to solve the above problems and provide a tray for carrying magnetoresistive heads of magnetic disks which has a stable surface resistivity in the range of from 104 to 1012 xcexa9/xe2x96xa1 and an even surface state considerably reduced in the generation of particles caused by scratching, wearing, or cleaning.
The tray of the invention for carrying a magnetoresistive head of magnetic disks is one obtained by molding of a resin composition comprising a thermoplastic resin material and carbon fibrils incorporated therein, and is characterized in that the carbon fibrils have a fiber diameter 100 nm or smaller and a fiber length/fiber diameter ratio of 5 or larger, and that the incorporation amount of the carbon fibrils is from 0.1 to 8 parts by weight per 100 parts by weight of the thermoplastic resin material.